PCOS, Metformin & Late Period

Courtney
I would love someone's personal experience, opinions!!
I'm on CD 37 now, usually my cycle is 31 days (like clock work). I do have PCOS & I'm Insulin Resistant so obviously it's not completely uncommon for a late period. I started metformin 500mg in July. I got a BFN on Monday (CD 35), with FMU. Going for blood HCG test tomorrow...which I'll probably take a HPT in the morning again. 
Has anyone else had this happen to them & actually be pregnant? Been TTC Baby #2 for 2 1/2 years now.